Optical fibers are key components in modern telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction. Optical fiber gratings are important elements for selectively controlling specific wavelengths of light within an optical fiber. Such gratings include Bragg gratings and long period gratings.
A typical Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction substantially equally spaced along the fiber length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10 .lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 .LAMBDA.' to 4/5 .LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the fiber core, long-period gratings remove light without reflection as by converting it from a guided mode to a non-guided mode. A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode. The spacing .LAMBDA.' of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is subsequently stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths of an optical communication system.
In the Bragg gratings, both n.sub.eff and .LAMBDA. are temperature dependent, with the net temperature dependence for a grating in silica-based fiber exemplarily being about +0.0115 nm/.degree. C. for .lambda.=1550 nm. The temperature-induced shift in the reflection wavelength typically is primarily due to the change in n.sub.eff with temperature. The thermal expansion-induced change in .LAMBDA. is responsible for only a small fraction of the net temperature dependence of a grating in a conventional SiO.sub.2 -based fiber.
In many applications of fiber Bragg gratings it would be highly desirable if the reflection wavelength were relatively temperature-independent. U.S. patent application Ser. No. 08/539,473, filed Oct. 4, 1995 by D. J. DiGiovanni et al. discloses relatively temperature insensitive long period fiber gratings. The temperature insensitivity is attained by appropriate selection of cladding composition in a fiber with multilayer cladding. See also the co-pending, co-assigned patent application Ser. No. 08/716,658 entitled "Long-Period Fiber Grating Devices Packaged for Temperature Stability" and filed on Jul. 6, 1996 by J. B. Judkins et al.
U.S. Pat. No. 5,042,898 discloses apparatus that can provide temperature compensation of a fiber Bragg grating. The apparatus comprises two juxtaposed compensating members that differ with respect to the coefficient of thermal expansion (CTE). Both members have a conventional positive CTE. The fiber is rigidly attached to each of the members, with the grating disposed between two attachment points. The apparatus can be designed to apply tensile or compressive stress to the grating. In the latter case the grating is confined in a small tube, exemplarily a silica tube.
The prior art designs are typically considerably longer than the grating, e.g. at least 40% longer than the grating device, thus making the temperature compensated package undesirably large. In addition, temperature compensating packages can have a substantial variation of reflection wavelength from one package to another because of the variability in the grating periodicity as well as minute variations in the degree of pre-stress applied to each grating or minute variations in the attachment locations.
Accordingly, there is a need for compact packaging design for temperature compensating fiber grating devices. There is also a need for such a design with simple and easy fine adjustment of the grating wavelength.